Common to the manufacture of all electronic devices is the formation of electrically conductive connections and bonds between cooperating microelectronic components. It is important, indeed critical, to device performance that the electrically conducting interconnections of device components are made as dependable as the device components themselves. Yet there are many practical considerations, not the least of which is manufacturing cost, which often dictate the nature of the process steps on the assembly line. Electronics device manufacturers and their suppliers have invested in a significant ongoing research and development effort to define new materials and processes for forming reliable electrically conductive connections for electronic device manufacture. The goal of many of such efforts has been to define new conductive adhesives that exhibit good processability and provide reliable product performance.
Two product areas that have been the target of significant research and development efforts are conductive adhesives for die attach (chip bonding) applications and solder paste used for board-level packaging--surface mount technology. Industry manufacturing practices for die attach include the use of thermoplastic (solvent containing) die attach adhesives, thermoset epoxies (both low temperature--slow curing for batch operations--and snap-cure epoxies, and solvent-based bismaleimide systems and polyamide systems, among others. State-of-the-art die attach materials are applied using either stamping or pin transfer, dispense or screen printing equipment. The die are then placed in contact with the adhesive which is typically batch cured in a forced air, recirculating oven. If the die attach adhesive contains a solvent, care must be taken to slowly remove the volatiles below the cure temperature to minimize void formation in the bond line. Such does not allow use of the more production efficient in-line automation for die attachment processes. While in-line curing has been detailed for certain snap-cure epoxy die attach adhesives, use of such compositions suffers the disadvantage of high rate of stress-related device failure, (especially for large chip manufacture) and corrosion due to extractable ionic species. There is clearly a significant need for the development of snap-cure conductive adhesives which avoid the known disadvantages of available snap-cure epoxies and other state-of-the-art die attach formulations.
Solder paste is a soldering material in paste form that readily adapts to automation, and accordingly has become the primary interconnecting material for board-level packaging. Solder paste is a mixture of a fusible metal powder with fluxes, activators, solvents, binders and other ingredients designed to give the paste its targeted rheology. The tacky characteristic of solder paste allows parts to be held in position without additional adhesives before permanent electrically conductive bonds are formed in the solder reflow process. The rheological properties of solder paste are such that it can be applied by mesh screen printing, metal stencil printing, pneumatic dot dispensing, positive displacement dispensing and pin transfer techniques. Metal mask stencil has become a popular means for solder paste application because of its compatibility with solder pattern and performance/maintenance advantages. Yet the use of solder paste for electrical conductive bonding is not without its disadvantages. Differential expansion between bonded surfaces leads to stress on solder joints leading to joint cracking and concomitant device failure. Further the use of solder paste in manufacturing operations typically results in high waste--the pastes are not rheologically stable when exposed to the air and therefore have short "open times". Significantly, too, there are environmental concerns associated with the high volume use of solder paste. The paste typically contain high levels of toxic metals, including lead. Further, following solder reflow, most manufacturing processes require cleaning of solder flux contaminants, which involve additional labor costs and capital equipment. Solvent cleaning is sometimes required; there are many environmental/health issues associated with the handling of chlorinated and fluorinated solvents.
Recognition of the disadvantages along with the high volume use of solder paste has provided significant incentive to develop solder paste replacement formulations. Paralleling the development of electrically conductive epoxy adhesives for die attach applications, metal filled conductive epoxies have also been proposed as solder paste replacements. Yet epoxy-based solder replacement formulations suffer from the same disadvantages, although of a lesser significance, seen with their use in die attach adhesives, and accordingly have not been accepted for production in industry. There is still a significant need in the electronics manufacturing industry to develop reliable, electrically conductive bonding materials which not only provide good performance properties but as well exhibit cost saving processability advantages.
Therefore, it is one object of this invention to provide a thermosetting resin formulation that can be thermally cured in in-line processing operations to provide a low stress, electrically conductive resin matrix.
It is another object of this invention to provide conductor-filled thermosetting resins which due to their free radical based cure chemistry not only exhibit good rheological stability at ambient temperature but can be induced to cure rapidly at temperatures of 150.degree.-200.degree. C.
Another object of this invention is to provide a thermosetting resin formulation that can be used as a solder paste replacement in surface mount printed circuit board manufacturing processes.
It is still another object of this invention to provide a thermosetting resin formulation utilizing a free radical propagated cure mechanism as a die attach adhesive.
Another object of this invention is to provide an improved method for electrically conductive attachment of surface mount electronics components to printed circuit boards utilizing a thermally initiated, free radical based curing conductor-filled resin formulation.
One other object of this invention to provide a die attach adhesive that can be fully cured in an in-line process with much reduced device failure due to stress failure.
Those and other objects of this invention are achieved by a conductor-filled thermosetting resin formulation comprising a free radical cross linkable polymer, a free radical initiating agent, and an unsaturated monomer capable of cross-linking the polymer. The cross-linking unsaturated monomer is preferably a bifunctional olefinic monomer having a boiling point greater than 200.degree. C. at atmospheric pressure. The thermosetting resin of this invention optionally includes a low shrink or low profile thermoplastic resin to reduce shrinkage and concomitant internal stress in the cured resin. The resin formulations can be cured to form a shape-retaining electrically conducting resin matrix in less than 5 minutes, more typically less than 2 minutes, at a temperature of 200.degree. C. or below. Physical and chemical properties of the resin formulations allow them to be used for die attachment and surface mount applications with little or no revision to existing manufacturing protocols.